The trend to substitute glass with other transparent plastics, also known as organic glass, has become widespread. Transparent windows are now utilized in transportation vehicles which have higher impact and shatter resistance due to the use of this synthetic organic glass. Optical lenses and especially ophthalmic eyeglasses often use organic glass to obtain lightweight, yet high impact resistant lenses.
One widely used transparent material is the thermoplastic polycarbonate such as Lexan® sold by General Electric Company, which provides major advantages over ordinary glass such as impact resistance, light weight and high refractive index. Polycarbonate has become widely used to replace glazing glass in transportation vehicles, building windows and optical lenses such as ophthalmic eyeglasses. One major drawback of polycarbonate is that the material is easily scratched and therefore typically requires surface protection for articles made from such polycarbonates.
Polycarbonate substrates are usually coated with a thin protective film to reduce their tendency to be abraded, resist corrosion, and to provide a sacrificial surfaces. It is also generally desirable that protective coatings have good weathering and adhesion properties. It is also desirable that such coatings be resistant to thermal shock, mechanical shock, heat, humidity, and common chemicals. In addition, the coatings must be practical to prepare, apply, dry, and cure.
Much effort has been exerted in this field, and several different technical approaches have been described. In particular, work has been carried out on the development of polyorganosiloxanes crosslinked by the condensation of silanol groups. The coatings described by the prior art, however, have numerous drawbacks such as poor adhesion or short pot life.
It is known in the art that polyorganosiloxane coating compositions with a basic pH such as those with amine functional compounds may have improved adhesion to plastic substrates. The resulting coating solutions, however, tend to increase viscosity over time and have a relatively short pot life. Pot life is known in the art as the time interval between completely mixing the coating elements together and the time when the coating solution becomes too viscous to satisfactory apply. In some circumstances, the pot life is known in the art as the gel time, or the time interval between complete mixing of the coating solution and gelation of the coating solution.
It is also known in the art that polyorganosiloxane coating compositions with an acidic pH, especially from pH of 3 to 5, result in more stable coating solutions. However, the application of such coatings to plastic substrate often requires the use of a primer prior to application of the coatings.
Some of these techniques can be seen in the prior art patents discussed below. The contents of each of these patents are herein incorporated by reference.
Mayazumi, in U.S. Pat. No. 3,837,876, describes a polymer formed by reacting an aminosilane with an epoxysilane, dissolving the resulting product in a solvent, and then coating various substrates with the solution of the product. Ender, in U.S. Pat. No. 3,166,527, describes the mixing of an epoxysilane with an aminosilane, then coating surfaces with both the unreacted mixture and the reacted (partially polymerized) mixture. The coating is cured by standing at room temperature for a longer period of time or by heating for a shorter period of time. The coatings described, however, would gel at room conditions within hours.
Treadway and Carr, in U.S. Pat. No. 4,378,250, describe the use of aldehydes or ketones as blocking agents in polymeric compositions derived from certain aminosilanes and epoxysilanes. The reference also describes the nuance of increasing the hydrolysis of the silanes to above 40%. The reference describes greater abrasion resistance in the cured product and longer pot life in the curable composition because of the presence of the ketone acting to retard the reaction between the amine functionality and the epoxy functionality on the various reactants. However, the coating solution still gels within a short time, for example a few days. Further, the required method of preparation can be long and is further restricted by a limited dye tintability range that can be obtained by varying the ratio of epoxy to amino within the bounds of the compositions described for attaining the desired level of abrasion resistance. Replication of these compositions shows crosslink equivalent weights of at least about 173 when fully cured.
U.S. Pat. No. 6,057,040 describes an alkine bridged bis-aminosilanes that may form a crosslinked polymeric coating. At least one of the polymerizable compounds comprises an alkine-bridged bis-(aminosilane) and another preferred polymerizable compound in the composition comprises an epoxy-functional silane. The coatings provided from these compositions are highly crosslinked and therefore display excellent mar resistance as well as increased tintability. As noted in the art, this combination is unusual since where one of these properties increases, it is done at the expense of the other property.
U.S. Pat. No. 6,346,331 B2 describes an epoxy bridged polyorganosiloxane coating compositions containing a glycidoxy functional silane, a tetrafunctional silane and a multi-functional organic compounds selected from the group consisting of multifunctional carboxylic acids, multifunctional anhydrides and combinations thereof. The coatings provided from these compositions have excellent abrasion resistance. However, these coatings do not adhere to polycarbonate substrates and therefore require the use of a primer prior to the application of the coatings.
U.S. Pat. No. 6,607,590 describes an organic-inorganic hybrid polymer arising from tetraalkyl orthosilicate, epoxyalkylalkoxy silanes, (math)acryaloxyalkylalkoxy silanes and solvent, coatings made from those coating compositions, and articles made from these coatings. The coatings have high scratch resistance based on a Bayer test. However, these coatings are described as not providing satisfactory adhesion to high index and polycarbonate lens materials. An activator or primer is typically required to apply these coatings to high index and polycarbonate lens material.
Other U.S. Pat. Nos. 3,976,497, 3,9886,997, 4,027,073 described mar-resistant coating compositions formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes such as alkoxy silanes in water or alcohol medium. These polysilicic coatings, especially when acidic, don't have good adhesion to many plastic, especially polycarbonate substrates.
In U.S. Pat. No. 4,413,979 Frye suggests the use of aggressive solvents such diacetone alcohol to improve adhesion. The solvents, however, are typically not sufficient to provide adequate adhesion to some plastic materials such as polycarbonate.
U.S. Pat. No. 4,783,347 introduced a method to improve adhesion of protective silicone coatings to plastic without the use of primers by shock curing the coated substrates at a temperature not substantially below 140 C. The shock curing temperature, however, is very close to the glass transition temperatures (Tg) of many plastic materials. For example polycarbonate has Tg of 150 C. The shock curing then can affect the optical properties of the articles produced and optical applications are where polycarbonate is used most.
It is an unexpected result to the inventor that when adding tertiary amines into polyorganosiloxan coating compositions at acidic conditions, the adhesion of the coating to many plastic, especially polycarbonate substrates, are improved substantially. The resulting coating solutions, however, still have sufficient pot lives. The presence of the tertiary amines also increases abrasion and scratch resistance of the resulted coatings.